Charge-domain pipelined charge-redistribution analog-to-digital converter

ABSTRACT

A charge-domain pipeline of at least two stages is provided. Each stage comprises a charge-storage node, a charge-transfer circuit for conveying charge from said charge-storage node out of said stage, a charge-control capacitor connected to said charge-storage node and driven by a periodic clock voltage, a comparator which compares the voltage of said charge-storage node to a reference voltage, and a digital latch which latches the state of said comparator output under control of a second periodic clock voltage and provides a latched digital output from said stage. The second stage of the pipeline further includes a first charge-redistribution capacitor connected to the charge-storage node of the second stage and driven by a conditional voltage responsive to the latched digital output from the first stage. The charge output from each stage of said pipeline is substantially similar to the charge input to said stage.

RELATED APPLICATION(S)

This application is a Continuation of U.S. application Ser. No.12/074,706, filed Mar. 5, 2008, now U.S. Pat. No. 7,683,820, whichclaims the benefit of U.S. Provisional Application No. 60/904,953, filedon Mar. 5, 2007. The entire teachings of the above applications areincorporated herein by reference.

BACKGROUND OF THE INVENTION

In charge-domain signal-processing circuits, signals are represented ascharge packets. These charge packets are stored, transferred from onestorage location to another, and otherwise processed to carry outspecific signal-processing functions. Charge packets are capable ofrepresenting analog quantities, with the charge-packet size in coulombsbeing proportional to the signal represented. Charge-domain operationssuch as charge-transfer are driven by periodic clock voltages, providingdiscrete-time processing. Thus, charge-domain circuits provide analog,discrete-time signal-processing capability. This capability iswell-suited to performing analog-to-digital conversion using pipelinealgorithms. Charge-domain circuits are implemented as charge-coupleddevices (CCDs), as MOS bucket-brigade devices (BBDs), and as bipolarBBDs. The present invention pertains to MOS and bipolar BBDs.

Charge-redistribution analog-to-digital converters (ADCs) implement asuccessive-approximation A/D conversion algorithm by iterated comparisonand conditional capacitor switching in a single stage. They are capableof providing very high precision and resolution (up to 18 bits incurrently-known implementations) at relatively low sample rates, withlow power consumption.

Pipelined analog-to-digital converters (ADCs) are commonly used inapplications requiring high sample rates and high resolution. PipelinedADCs implement the successive-approximation algorithm by resolving oneor several bits at each pipeline stage, subtracting the quantizedestimate from the signal at that stage, and propagating the residue tothe next pipeline stage for further processing. Pipelined ADCs have beenimplemented using a variety of circuit techniques, includingswitched-capacitor circuits and charge-domain circuits. The presentinvention pertains to charge-domain pipelined ADCs.

SUMMARY OF THE INVENTION

Typical charge-redistribution ADCs resolve a sampled input signalvoltage by implementing a successive-approximation algorithm in a singlestage, requiring multiple clock cycles to process each sample.Consequently the sample rate of such an ADC is limited to a maximum of1/N^(th) of the clock rate, where N is the resolution in bits of theADC. Power consumption of charge-redistribution ADCs for a given samplerate can approach the theoretical minimum for A/D conversion.

Pipelined ADCs provide higher sample rates, by resolving only a few bitsat each stage, then propagating an unresolved residue signal to the nextstage for further processing. Typically such converters acquire onesample of the input signal per clock cycle, thus providing approximatelyN-times the sample rate of charge-redistribution ADCs. They consumerelatively large power, however, because of the use of active circuitry,typically op-amps, for the pipelining operation. Moreover, each suchpipelining operation adds error components to the propagating residues,limiting the available resolution and linearity of such ADCs.

In one embodiment, an analog-to-digital converter is implemented in atleast two stages. The first stage has a first charge storage device, afirst comparator, a first digital charge storage device and a firstcharge transfer device. The first charge storage device stores an inputcharge that corresponds to an analog input signal. The first comparatorcompares reference voltage against a voltage corresponding to the inputcharge as stored at the first charge storage device, to provide a firstcomparison signal. The first digital storage device, in turn, stores thefirst comparison signal as the first digital output signal. The firstcharge transfer device also transfers the stored input charge out of thefirst stage.

The second stage has a second charge storage device, a chargeredistribution device, a second comparator, and a second digital storagedevice. The second charge storage device stores the stored input chargeas received from the first charge transfer device of the first stage.The charge redistribution device provides a redistribution charge thatdepends on the first comparison signal. The second comparator compares areference voltage with the stored input charge, and the redistributioncharge, to provide a second comparison signal. Finally, the seconddigital storage device is for storing the second comparison signal toprovide a second digital output signal.

In other embodiments, a method for converting an analog input to digitaloutputs proceeds as follows. An input charge is stored at a first chargestorage device, the input charge corresponding to the analog input.Charge stored as the input charge is then compared to a reference, toprovide a first comparison result. The first comparison result is inturn used as a first digital output. The input charge is alsotransferred to a second charge storage device along with aredistribution charge to a third charge storage device. The amount ofredistribution charge depends on the first step of comparing. Finally, asecond step of comparing charge, as stored by the second charge storagedevice and the third charge storage device, is performed against areference. This then provides a second comparison result which is usedas a second digital output.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing will be apparent from the following more particulardescription of example embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingembodiments of the present invention.

FIG. 1 shows a simplified circuit diagram of a BBD charge-pipelinestage.

FIG. 2 illustrates voltage waveforms associated with FIG. 1.

FIG. 3 shows a two-stage BBD charge pipeline.

FIG. 4 illustrates voltage waveforms associated with FIG. 3.

FIG. 5 shows a BBD charge-pipeline stage including conditional chargeredistribution.

FIG. 6 illustrates voltage waveforms associated with FIG. 5.

FIG. 7 shows a BBD charge-pipeline stage with two independentcharge-redistribution capacitors.

FIG. 8 shows a single-ended BBD charge-pipeline stage including chargecomparison.

FIG. 9 shows a differential BBD charge-pipeline stage including chargecomparison.

FIG. 10 shows a single-ended BBD charge-pipeline stage includingconditional charge redistribution and charge comparison.

FIG. 11 shows a two-stage ADC according to the present invention.

FIG. 12 illustrates voltage waveforms associated with FIG. 11.

FIG. 13 shows a three-stage ADC according to the present invention.

FIG. 14 shows a differential two-stage ADC according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

A description of example embodiments of the invention follows. Theteachings of all patents, published applications and references citedherein are incorporated by reference in their entirety.

Prior-art charge-redistribution ADCs have been implemented in a singlestage, in which the signal input voltage is sampled onto a common nodeshared by an array of capacitors. The capacitors are then sequentiallyswitched between reference voltages, in accordance with asuccessive-approximation algorithm. This capacitor-switching processre-distributes the sampled charge among the various capacitors of thearray, resulting in a voltage change at the common node. At each step ofthe algorithm, the resulting common-node voltage is compared against areference voltage, with the comparison result governing the nextcapacitor-switching event. This A/D conversion process is limited insample rate, because the entire multi-step successive-approximationprocess must be completed for a given sample before the next sample isacquired.

In prior-art pipelined ADCs, each stage carries out operations similarto those in a charge-redistribution ADC, i.e., comparison andconditional capacitor switching. However, instead of carrying out allsteps of the successive-approximation algorithm in a single stage, as isdone in charge-redistribution ADCs, a pipelined ADC resolves only asmall number of bits in each stage (typically in a single clock cycle),then subtracts charge corresponding to the resolved bits and propagatesthe residue to the next stage. Successive steps of A/D conversion, witha few bits resolved at each step, are carried out in successive pipelinestages.

Embodiments of the present invention combines aspects of these two ADCarchitectures in a charge-domain implementation. In order to make clearits operating principle, the basic principles of a charge-domainpipeline are first described. The ADC of the present invention is thendescribed by reference to this pipeline description.

The charge-domain pipelines employed in the present invention are of thebucket-brigade device (BBD) type. Such pipelines employ charge-transfercircuits to convey charge from each stage to the next. MOS and bipolarimplementations of these circuits, using either electrons or holes asthe signal-charge carriers, are known. Some examples of such circuitsare described in a previous patent application by the same inventor(U.S. Provisional Patent Application No. 60/809,485, filed May 31, 2006entitled “Boosted Charge Transfer Pipeline”). In the followingdescription, the charge-transfer circuits are treated as abstractobjects, since details of their operation are not pertinent to thepresent invention. Negative charge-carriers (electrons) are assumed forpurposes of discussion.

The basic principle of a BBD pipeline of the general type employed inthis invention is described with the aid of FIG. 1, which depicts asingle stage of such a pipeline. In this stage charge is stored oncapacitor 5, which is connected between storage node 2 and voltageV_(C1). Charge enters the stage via charge-transfer circuit 1, and laterexits the stage via charge-transfer circuit 3. Voltage V_(C1) is adigital clock signal which controls the timing of charge processing inthe stage. Other digital clock signals, not shown, may be used tocontrol the activity of the charge-transfer circuits.

Operating waveforms of the pipeline stage are shown in FIG. 2. At timet₀ clock voltage V_(C1) has a positive value 25. V₂, the voltage ofstorage-node 2 in FIG. 1, is also at a high initial voltage 21. At t₁negative charge begins to be transferred from the previous stage (to theleft of FIG. 1) via charge-transfer circuit 1 into the stage shown. Asthis negative charge accumulates on capacitor 5, V₂ falls to a morenegative value. The voltage of node 2 settles to a relatively high value22A if a relatively small negative charge was transferred; with a largercharge transferred, node 2 settles to a more negative voltage 22B. Attime t₂ charge transfer into the stage is complete. The voltage of node2 is related to the charge by the well-known expression Q=CV, where isthe total capacitance of node 2. In FIG. 1, C is comprised of C₅, thecapacitance of capacitor 5, plus any parasitic capacitance of node 2;such parasitic capacitance is usually small and is neglected in thisdiscussion.

Charge transfer out of the stage begins at time t₃ when clock voltageV_(C1) switches to a low state, voltage 26. Capacitor 5 couples thisvoltage transition to node 2, driving V₂ low as well. Charge-transfercircuit 3 absorbs charge from capacitor 5, limiting the negativeexcursion of node 2, and eventually causing node 2 to settle to voltage23 at t₄. Voltage 23 is a characteristic of charge-transfer circuit 3,and is independent of the amount of charge which had been stored on node2. Charge-transfer circuit 3 transfers the charge absorbed fromcapacitor 5 to node 4 which is part of the stage following the oneshown. After t₄ charge transfer is complete.

Finally, at time t₅, clock voltage V_(C1) returns to its initial state(voltage 25). Its positive-going transition is coupled to node 2 bycapacitor 5, raising node 2 to voltage 24. Neglecting parasiticcapacitance, no charge flows onto or off of node 2 during thistransition; the voltage change of V₂ is therefore equal to the voltagechange of V_(C1) during the transition at t₅. Since V₂'s value at thestart of this transition, voltage 23, is independent of chargeprocessed, voltage 24 is likewise independent of charge processed. Thistransition completes the operating cycle; the resulting voltage 24 atnode 2 is thus the initial voltage for the next cycle. Thus the initialvoltage state of the stage is constant cycle-to-cycle, and voltage21=voltage 24. Consequently the initial and final charge on node 2 arealso equal, and the charge transferred out is equal to the chargetransferred in.

In summary: charge is transferred into the stage shown in FIG. 1 duringt₁-t₂; between times t₂ and t₃ it is temporarily stored on capacitor 5,and is manifested as the value of V₂; during times t₃-t₄ this charge iscompletely transferred to the next stage; at t₅ the stage returns to itsinitial state, ready again to receive incoming charge. Thus the basicstage shown acts as a shift register for analog charge packets.

It should be understood that practical circuits depart in variousdetails from this idealized description. Such departures includenon-zero parasitic capacitance and imperfect charge transfer, forexample. These effects, however, do not change the basic operatingprinciples described above; and these principles can be applied inpractical circuits with sufficient accuracy for useful purposes.Consequently such non-ideal effects are not considered in the followingdescriptions.

Conventional BBD charge pipelines have generally employed simpletwo-phase digital clock signals which simultaneously control thecharge-storage capacitors and the charge-transfer circuits. Pipelinecircuits such as that of FIG. 1 and others described below also operateusing a two-phase clocking system. In these circuits, however, it isdesirable to provide independent control of the activity of thecharge-transfer circuits and of other clocked events in the stage suchas capacitor switching. For this reason, the circuits of the presentinvention employ additional clock signals which control charge-transfercircuit activity. These signals and their function will be explainedwith the aid of FIGS. 3 and 4.

FIG. 3 shows a pipeline segment containing two successive stages, eachlike the basic pipeline stage of FIG. 1. This pipeline segment consistsof first charge-transfer circuit 31, part of the previous pipeline stagewhich is not shown; first storage node 32, first capacitor 35, andcharge-transfer circuit 33, together comprising first pipeline stage 38;and second storage node 34, second capacitor 36, and charge-transfercircuit 37, together comprising second pipeline stage 39. Clock voltagesV_(C1) and V_(C2) drive the two capacitors respectively; and digitalclock signals S_(CT1) and S_(CT2) control the charge-transfer circuits.

The waveforms associated with the operation of the circuit of FIG. 3 areshown in FIG. 4. The waveforms pertaining to first stage 38 in FIG. 3,V₃₂ and V_(C1), are identical with those of V₂ and V_(C1) respectivelyin FIG. 2. The waveforms pertaining to second stage 39 in FIG. 3, V₃₄and V_(C2), are similar, but shifted by one-half of the clock cycle fromthose of the first stage. Thus the two stages of FIG. 3 operate onalternate half-cycles of the complete clock cycle. During the firsthalf-cycle shown, when charge is transferring via charge-transfercircuit 31 into first stage 38 in FIG. 3, charge is transferring viacharge-transfer circuit 37 out of second stage 39 (into the next stage,not shown). Likewise, during the second half-cycle, while charge istransferring out of first stage 38 via charge-transfer circuit 33, it isbeing transferred into second stage 39.

In order to control the direction of charge transfer, it is necessary toselectively enable the appropriate charge-transfer circuits. The digitalsignals S_(CT1) and S_(CT2) provide this control. As shown in FIG. 4,S_(CT2) is asserted (high) during the interval t₁-t₂. This controlsignal enables charge-transfer circuits 31 and 37, which are activeduring this interval as described above. During the correspondinginterval in the second half-cycle, t₃-t₄, S_(CT1) is asserted, enablingcharge-transfer circuit 33. The exact means by which the digital signalsS_(CT1) and S_(CT2) act to control the activity of the charge-transfercircuits is not pertinent to this invention. Some examples of suchcontrol are described in the aforementioned patent application (“BoostedCharge-Transfer Pipeline”).

The two-phase operating mode just described is used in all of thepipeline circuits described below, together with control (by signalsequivalent to S_(CT1) and S_(CT2)) of the charge-transfer circuits. Inthe interest of clarity, these details are not repeated in subsequentfigures or descriptions.

In order to form the ADC of the present invention from a pipelinecomposed of stages similar to FIG. 1, two operations in addition tocharge storage and charge transfer are required: charges must becompared to a reference value, typically another charge; and conditionalredistribution of charge on the storage nodes must be carried out. Inthe ADC of this invention, one or both of these operations are carriedout in each of several pipeline stages. Implementation of theseoperations is explained below, beginning with charge redistribution.

Charge redistribution, as employed in the ADC of this invention, isdescribed with reference to FIGS. 5 and 6. For the purposes of thisdiscussion, a single-ended stage is shown. In practical ADC designs,differential operation is usually preferred; both modes are possiblewithin the scope of this invention. The pipeline stage shown in FIG. 5retains all the elements shown in FIG. 1, and includes one new element:capacitor 6 (with value C₆) connected between charge-storage node 2 andvoltage V_(QR1). FIG. 6 shows the operating waveforms of the circuit ofFIG. 5.

The waveforms of FIG. 6 are identical to those shown in FIG. 2, exceptfor the time interval t_(3A)-t_(3B), discussed below. As in FIG. 2,charge is transferred into the stage between t₁ and t₂, causing V₂ tofall in proportion to the incoming charge, settling at voltage 42. Thechange in V₂ due to incoming charge is inversely proportional to thetotal capacitance of node 2, which in FIG. 5 is C=C₅+C₆. As in FIG. 2,charge transfer out of the stage begins at t₃, driven by the negativetransition of V_(C1). By t₄, V₂ settles to voltage 43 which isindependent of the charge previously on node 2, and charge transfer outof the stage is complete. At t₅ V_(C1) returns to its initial high state(voltage 45), completing the stage's operating cycle.

Between t_(3A) and t_(3B) the new features of FIG. 5 come into play.Voltage V_(QR1) is initially at voltage 47. At time t_(3A) voltageV_(QR1) conditionally switches from voltage 47 to voltage 48. The term‘conditionally’ as used here indicates that V_(QR1) either switches asstated or else it remains at voltage 47; these outcomes are shown asdashed and solid lines respectively in FIG. 6. This conditionaltransition of V_(QR1) is coupled via C₆ to node 2 where, because ofcapacitive division, it produces a similar but smaller voltage change.The voltage at node 2 changes to voltage 49 (dashed line) if V_(QR1)switches, and remains at voltage 42 (solid line) if it does not. At timet_(3B) V_(QR1) is unconditionally restored to voltage 47; consequentlyV₂ returns to voltage 42. Voltage V₂ is thus dependent on both inputcharge and on the (conditional) state of V_(QR1) between t_(3A) andt_(3B), but is dependent only on the input charge after t_(3B).

Applying the relationship Q=CV to the conditions of FIGS. 5 and 6, andemploying the notation v41 for “voltage 41”, v42 for “voltage 42”, etc.,we can express the voltage V₂ of charge-storage node 2 at t=t₂ as:V ₂ =v ₄₂ =v ₄₁ +Q _(IN) /C  Equation 1

where Q_(IN) is the incoming charge from the previous stage (negative inthis example), and C=C₅+C₆ is the total capacitance at node 2.

After the conditional transition of V_(QR1) (t>t_(3A)), the node-2voltage becomes:V ₂ =v ₄₂ +ΔV _(QR1) C ₆ /C=v ₄₁+(Q _(IN) +ΔV _(QR1) C ₆)/C  Equation 2

where ΔV_(QR1) is the change in voltage V_(QR1) at t_(3A), having thevalue either (v₄₈-v₄₇) or zero. The corresponding change in V₂,ΔV_(QR1)C₆/C, is due to capacitive division between capacitor 6 and thetotal capacitance of the node. This change in voltage due to capacitivedivision is alternatively known as ‘charge-redistribution’. Equation 2shows quantitatively the dependence of V₂ on both input charge and theconditional change in V_(QR1). In the following discussion,conditionally-switchable voltages such as V_(QR1) are referred to as‘charge-redistribution voltages’; the capacitors such as capacitor 6driven by these voltages are referred to as ‘charge-redistributioncapacitors’.

The charge-redistribution principle shown in FIGS. 5 and 6 and expressedin Equation 2 can be extended as shown in the circuit of FIG. 7. Thiscircuit is identical to that of FIG. 5, except that capacitor 6 has herebeen re-named 6-1, and a second charge-redistribution capacitor 6-2 andvoltage source V_(QR2) have been added. For convenience in the followinganalysis, the capacitances of capacitors 6-1 and 6-2 are represented asC₁ and C₂ respectively. Operation of this circuit is as shown in FIG. 6,except that V_(QR1) and V_(QR2) can be switched independently at t_(3A).(Both are returned to their original voltages, equivalent to voltage 47in FIG. 6, at t_(3B).) With these changes the total capacitance at node2 becomes C=C₅+C₁+C₂. Expressing the (independent) changes in V_(QR1)and V_(QR2) as ΔV_(QR1) and ΔV_(QR2) respectively, we obtain, byextension from Equation 2, the voltage at node 2 between times t_(3A)and t_(3B):

$\begin{matrix}{V_{2} = {{v_{42} + {\Delta\; V_{{QR}\; 1}{C_{1}/C}} + {\Delta\; V_{{QR}\; 2}{C_{2}/C}}}\mspace{25mu} = {v_{41} + {\left( {Q_{IN} + {\Delta\; V_{{QR}\; 1}C_{1}} + {\Delta\; V_{{QR}\; 2}C_{2}}} \right)/C}}}} & {{Equation}\mspace{14mu} 3}\end{matrix}$

This principle can be extended indefinitely, with as manycharge-redistribution capacitors and independently-switchable voltagesas desired. The resulting voltage at node 2 then resembles Equation 3,with one term of the form ΔV_(QRk)C_(k) for each such capacitor, and Cbeing the total capacitance. In the case of zero charge-redistributioncapacitors, as in FIG. 1, Equation 3 reduces to Equation 1, and thetotal stage capacitance is simply that of the single capacitor 5 in FIG.1.

In FIG. 5 above the quantity (v₄₈-v₄₇) is shown as negative. This signchoice is merely an example: Equations 2 and 3 (and the indicatedextension to more capacitors) remain valid for either choice of sign.Similarly, the individual values of ΔV_(QR1), ΔV_(QR2), etc. can havedifferent values or signs. Likewise, the voltage to which V_(QR1) etc.transition at t_(3B) need not be the original voltage (v₄₇ in theexamples given). It must, however, be a voltage which is unconditional;that is, the final value of V_(QR1) etc. after t_(3B) must beindependent of the transition at t_(3A).

The exact position of times t_(3A) and t_(3B) are not critical to theoperation of the circuits of FIGS. 5 and 7. The conditional transitionsof V_(QR1) etc. (t_(3A) in FIG. 6) can take place as early as t₁, andthe transition of V_(QR1) etc. to their unconditional final states(t_(3B) in FIG. 6) can take place as late as t₃. Equations 2 and 3become valid at the later of t₂ and t_(3A), and remain valid untilt_(3B).

The second operation required for construction of an ADC according tothis invention is charge comparison. FIG. 8 shows a circuit whichprovides charge comparison in a pipeline stage. The circuit of FIG. 8may be comparable to that of FIG. 1, with the addition of voltagecomparator 8 and latch 9. Comparator 8 compares the voltage of node 2with a reference voltage V_(RC). The voltage at node 2 after t₂ dependson the amount of charge transferred into the stage, as expressed inEquation 1. Because of this dependence, voltage comparator 8accomplishes a comparison of charge on node 2 vs. a reference. Latch 9captures the result of this comparison at a time between t₂ and t₃defined by the digital clock signal V_(CC1), and provides a digitaloutput voltage V_(B).

As was mentioned above, many practical charge-domain pipelined ADCsemploy differential circuitry. In such circuitry, signals arerepresented by pairs of charges whose difference is proportional to thesignal. This arrangement permits representation of bipolar signals withunipolar charge packets, and can also provide dynamic range andnoise-immunity benefits.

FIG. 9 illustrates a differential pipeline stage which is functionallysimilar to the single-ended stage of FIG. 8. The circuit of FIG. 9contains two charge pipelines, each identical to that of FIG. 1. Theupper pipeline contains elements 1A, 2A, 3A, 4A and 5A, equivalent toelements 1, 2, 3, 4, and 5 in FIG. 1. The lower pipeline containselements 1B through 5B, also equivalent to elements 1 through 5 ofFIG. 1. The latch 9 in this circuit serves the same function as in FIG.8. In this differential configuration, however, the comparator 8compares the voltages of the two charge storage nodes 2A and 2B, ratherthan comparing to a fixed reference as in FIG. 8. Thus the comparatordecision in FIG. 9 is based on the sign of the differential chargesignal during the t₂-t₃ interval.

Returning to a single-ended circuit for simplicity, FIG. 10 illustratesa pipeline stage in which the charge-comparison function just describedis combined with the charge-redistribution circuitry describedpreviously. The circuit of FIG. 10 combines the elements of FIG. 8 withthe charge-redistribution capacitor 6 and voltage V_(QR1) from FIG. 5.With this combination, comparator 8 senses the voltage resulting fromthe combination of input charge to the stage and charge redistribution,as given by Equation 2. As in the circuits of FIGS. 8 and 9, the clockvoltage V_(CC1) determines the time at which the comparator output islatched. This time is chosen to occur in the interval during whichEquation 2 is valid, as discussed above. A similar circuit combination,with an additional charge-redistribution capacitor as in FIG. 7,provides the comparator with an input signal given by Equation 3; andsimilarly for additional charge-redistribution capacitors.

In an ADC according to the present invention, each pipeline stage isprovided with a comparator and latch like those described above. Thedigital output signal from each such latch constitutes one bit of thecomplete ADC's digital output. In addition, each such digital output isused to control the conditional charge-redistribution in the followingpipeline stages, as will be shown below. In order to achieve thiscontrol, an additional circuit is required which accepts as input thedigital signal and provides as output a signal similar in logic sense,timing, and amplitude to V_(QR1) in FIG. 6. This circuit can beimplemented using well-known methods and will not be described indetail; it will be referred to in the following description as a‘charge-redistribution driver’ circuit.

FIG. 11 shows the first two stages of an ADC according to the presentinvention, which incorporates the charge-redistribution,charge-comparison, and charge-redistribution-driver circuits describedabove in a single-ended pipeline providing two bits of analog-to-digitalconversion. The first pipeline stage 111 may be comparable to the stageof FIG. 8, and is supplied with clock phases V_(C1) and V_(CC1). Stage112 is comparable to the circuit of FIG. 10, including acharge-redistribution capacitor driven by input voltage V_(QR1); stage112 is supplied with clock phases V_(C2) and V_(CC2). Both stages aresupplied with comparator reference voltage V_(RC). The latched digitaloutputs of stages 111 and 112 are provided as outputs V_(B1) and V_(B2)respectively. Charge-redistribution driver circuit 113 has V_(B1) asinput and V_(QR1) as output.

FIG. 12 shows operating waveforms of the circuit of FIG. 11. Thecharge-storage node voltages in each pipeline stage, identifiedrespectively as V₁₁₁ and V₁₁₂, are plotted in two separate panels as inFIG. 4, together with their respective charge-control clock phasesV_(C1) and V_(C2) and the (DC) comparator reference voltage V_(RC). Thetwo-phase clocking system depicted is identical to that shown in FIG. 4.Four clock half-cycles are shown, identified as 121, 122, 123 and 124.

The passage of two successive charge packets through the pipeline can bedescribed as follows. During the first clock half-cycle 121, a firstcharge packet is transferred into stage 111, causing storage-nodevoltage V₁₁₁ to fall to voltage 125A. During the second half-cycle 122,this charge packet is transferred from stage 111 into stage 112, causingV₁₁₂ to fall to the same level, identified as 125B. During the thirdhalf-cycle 123, the same charge is transferred out of stage 112 into thenext pipeline stage, which is not shown. Meanwhile, also duringhalf-cycle 123, a second charge packet is transferred into stage 111,causing storage-node voltage V₁₁₁ to fall to voltage 127A. During thefourth half cycle 124, the second charge packet is transferred intostage 112, causing its storage-node voltage V₁₁₂ to fall to the samevoltage, identified as 127B.

Charge comparison and redistribution occur as follows. Storage-nodevoltages V₁₁₁ and V₁₁₂ are compared to V_(RC); the comparison resultsare latched at the indicated times t_(CC1) and t_(CC2) respectively,producing digital outputs V_(B1) and V_(B2). (These latching times aregoverned by the respective latch clocks V_(CC1) and V_(CC2) shown inFIG. 11. They recur in each full clock cycle.) During half-cycle 121,the result of comparing V₁₁₁ to V_(RC) is latched at t_(CC1), resultingin a high state (digital 1) for V_(B1) as shown. The value of V₁₁₁sensed at this time is a function of the input charge, as given byEquation 1.

As shown in FIG. 11, charge-redistribution driver 113 receives V_(B1)and outputs a charge-redistribution voltage signal V_(QR1) which isinput to stage 112. The sense of this V_(QR1) signal (inverted fromV_(B1)) and its timing (delayed from V_(B1)) are shown in FIG. 12. Asexplained above, the transition in V_(QR1) causes a corresponding changein V₁₁₂, resulting temporarily in voltage 126. The V_(QR1) timing shownin FIG. 12 illustrates one feature of the present invention: thecomparison result of a given stage (111 in this example) governscharge-redistribution in subsequent stages such as 112.

At t_(CC2) in half-cycle 122 the result of comparing V₁₁₂ with V_(RC) islatched, resulting in the indicated V_(B2) value (also a digital 1 inthis example). Note that the V₁₁₂ voltage whose value governs thisV_(B2) decision includes the effects of both the input charge packet andthe effect of V_(QR1) switching, as given by Equation 2. If V₁₁₂ weremore negative than V_(RC), then V_(B2) would be a digital 0.

The second charge packet is transferred into stage 111 during half-cycle123, resulting in V₁₁₁ voltage 127A. Since v_(127A)<V_(RC), theresulting V_(B1) value latched at t_(CC1) is a digital 0, as shown inFIG. 12. Consequently, when this second charge packet is transferredinto stage 112 (resulting in V₁₁₂ voltage 127B), V_(QR1) does not make anegative transition, and V₁₁₂ remains at voltage 127B throughouthalf-cycle 124. Since v_(127B)<V_(RC), the resulting V_(B2) valuelatched at t_(CC2) in this half-cycle is a also digital 0.

FIG. 13 illustrates the extension of the concepts discussed above tothree stages. In addition to the elements shown in FIG. 11, it includesa third pipeline stage 114, controlled (like stage 111) by clock phasesV_(C1) and V_(CC1); two additional charge-redistribution drivers, 115and 116; and latch 117. Pipeline stage 114 includes twocharge-redistribution capacitors, controlled by twocharge-redistribution voltages, V_(QR2) and V_(QR3). V_(QR2) is theoutput of charge-redistribution driver 115, whose input is V_(B2), thebit resolved in stage 112. V_(QR3) is the output ofcharge-redistribution driver 116, whose input is signal 118, the outputof latch 117, whose input is V_(B1), the bit resolved in stage 111.Stage 114 provides V_(B3) as its digital output bit.

The operation of the circuit of FIG. 13 is as follows. The first twostages, 111 and 112 operate exactly as described above. Charge packetsare received as input to stage 111, transferred in turn to stage 112, tostage 114, and then out of stage 114 to the next pipeline stage (notshown). Charge redistribution in second stage 112 is governed by the bitdecision made in first stage 111: the first-stage output bit, V_(B1), isdelayed and inverted by charge-redistribution driver 113 to produceV_(QR1), as described above. Similarly, one charge-redistributioncapacitor in third stage 114 is controlled by the bit decision made insecond stage 112: the second-stage output bit, V_(B2), is delayed andinverted by charge-redistribution driver 115 to produce V_(QR2). Theother charge-redistribution capacitor in third stage 114 is controlledby a delayed version of V_(B1): latch 117 captures the state of V_(B1)at t_(CC2) (at the same time as V_(B2) is being latched) to producedigital signal 118. Signal 118 in turn provides input tocharge-redistribution driver 116, whose output is V_(QR3).

This effect of this operation is as follows. A charge packet istransferred in to first stage 111 where the resulting voltage iscompared to reference V_(RC), with the comparison result provided asoutput V_(B1). The charge packet is then transferred to second stage112. In stage 112, a reference charge is temporarily added to the chargepacket if V_(B1) indicates that the packet is smaller than the(charge-equivalent) threshold. The combined charge in stage 112 is thencompared to the same reference, resulting in output V_(B2) which isprovided as the second-stage output. The charge packet is thentransferred to third stage 114. Note that the charge transferred isequal to the original input packet: the temporarily-added referencecharge is removed before the transfer occurs. In stage 114, the twoprevious decisions regarding the same packet control the conditionaladdition of two independent reference charges. V_(B2) is available whenneeded for this purpose. Latch 117 serves to propagate the first-stagebit decision (V_(B)) so that it is available when needed in third stage114. The voltage resulting from the original charge packet plus the twoconditional added charges in stage 114 is compared to reference V_(RC),with the result provided as output V_(B3). The net result is that ateach stage, a sequentially augmented version of the input charge iscompared to a fixed threshold; the bit decisions at each stage governthe conditional charge addition at all downstream stages. Thisfunctionality is suitable for implementing the well-knownsuccessive-approximation A/D conversion algorithm, with the sequentialoutput bits V_(B1), V_(B2), and V_(B3) being the digital result of theconversion, MSB first.

This three-stage ADC example illustrates the principal features of thepresent invention; the same principles can be applied to four or morestages by obvious extension. Each stage includes one morecharge-redistribution capacitor than the previous one. Bit decisions aredelayed at each stage by latches analogous to 117, so that at each stagethe complete set of bit decisions from previous stages is available forcharge-redistribution control.

In addition to the single-ended embodiment described above, the ADC ofthis invention can be embodied using a differential pipeline similar tothe circuit of FIG. 9. As an example, FIG. 14 shows a differentialimplementation of the two-stage ADC circuit of FIG. 11. Stages 141 and142 are analogous to stages 111 and 112 in FIG. 11. The clocking(V_(C1), V_(CC1), V_(C2), and V_(CC2)) is identical to that in FIG. 11.The comparators 146 and 147 are similar in function to the comparatorsin FIG. 11, except that the comparison in FIG. 14 is between thedifferential charge-storage node voltages rather than between a singlecharge-storage node and a reference voltage. The first-stage output bitV_(B1) controls conditional charge addition in the second stage as inFIG. 11. In FIG. 14, however, the two charge-redistribution drivers 144and 145 receive complementary logic inputs: driver 144 receives V_(B1)directly, while driver 145 receives its complement, provided by logicinverter 143. Thus the two logic states of V_(B1) cause conditionalcharge addition either to the upper storage node or the lower storagenode (but not both) at each cycle. This differential embodiment can beextended to further stages by analogy to the extension from FIG. 11 toFIG. 13.

While this invention has been particularly shown and described withreferences to example embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

1. An analog-to-digital converter apparatus comprising: a first stagecomprising: a first charge storage device, for storing an input chargethat corresponds to an analog input signal; a first comparator, forcomparing a reference voltage against a voltage corresponding to theinput charge as stored at the first charge storage device, and inresponse thereto, providing a first comparison signal; a firstcomparison result storage device, for storing the first comparisonsignal and to provide a first digital output signal; and a first chargetransfer device, for transferring the stored input charge out of thefirst stage; a second stage comprising: a second charge storage devicefor storing the stored input charge as received from the first chargetransfer device of the first stage, a charge redistribution device, thecharge redistribution device providing a redistribution charge thatdepends on the first comparison signal; a second comparator, forcomparing a reference voltage with the stored input charge and theredistribution charge, to provide a second comparison signal; and asecond comparison result storage device, for storing the secondcomparison signal and to provide a second digital output signal.
 2. Theapparatus of claim 1 wherein at least one of the first and second chargestorage devices is a capacitive circuit element.
 3. The apparatus ofclaim 2 wherein the charge redistribution device comprises a capacitivecircuit element.
 4. The apparatus of claim 3 wherein the chargeredistribution device further comprises a driver circuit coupled to thecapacitive circuit element.
 5. The apparatus of claim 1 wherein thesecond charge storage device is a capacitor having a first terminalcoupled to receive the input charge and a second terminal coupled toreceive a clock signal.
 6. The apparatus of claim 1 wherein the firstcomparison result storage device is a clocked digital latch storing afirst bit of an analog-to-digital conversion result.
 7. The apparatus ofclaim 1 wherein the second comparison result storage device is a clockeddigital latch storing a second bit of an analog-to-digital conversionresult.
 8. The apparatus of claim 1, further comprising: a third stagecomprising: a third charge storage device for storing the stored inputcharge as received from the second charge transfer device of the secondstage; a second charge redistribution device, the second chargeredistribution device providing a second redistribution charge thatdepends on the first and second comparison signal from, respectively,the first and second comparator in the first and second stages; a thirdcomparator, for comparing a reference voltage with the stored inputcharge and the second redistribution charge, to provide a thirdcomparison signal; and a third comparison result storage device, forstoring the third comparison signal and to provide a third digitaloutput signal.
 9. The apparatus of claim 8 wherein the second chargeredistribution device further comprises: a second charge redistributioncapacitor coupled to a second common node, the second chargeredistribution capacitor providing a charge that depends on the firstcomparison signal; and a third charge redistribution capacitor, coupledto the second common node, the third charge redistribution capacitorproviding a charge that depends on the second comparison signal.
 10. Amethod for converting an analog input to digital outputs comprising:storing an input charge corresponding to the analog input, to provide astored input charge; a first step of comparing the stored input chargeto a reference, to provide a first comparison result; determining afirst digital output from the first comparison result; determining aredistribution charge depending on the first comparison result;combining the redistribution charge and the first comparison result toprovide a combined charge; a second step of comparing the combinedcharge against a reference, to provide a second comparison result and;determining a second digital output from the second comparison result.11. The method of claim 10 further comprising: storing the firstcomparison result in a digital storage device.
 12. The method of claim11 further comprising: storing the second comparison result in a digitalstorage device.
 13. The method of claim 10 wherein the step of storingan input charge further comprises storing the input charge in a firstcapacitive circuit element.
 14. The method of claim 13 wherein the stepof storing an input charge further comprises clocking the firstcapacitive circuit element.
 15. The method of claim 10 wherein the stepof transferring the input charge to a second charge storage devicefurther comprises storing the input charge in a second capacitivecircuit element.
 16. The method of claim 15 wherein the step oftransferring the input charge to a second charge storage device furthercomprises clocking the second capacitive circuit element.
 17. The methodof claim 16 wherein the step of determining a redistribution chargefurther comprises storing the input charge in a third capacitive circuitelement.
 18. The method of claim 17 wherein the step of determining aredistribution charge further comprises coupling the second and thirdcapacitive circuit elements to a common node.
 19. The method of claim 18wherein the second step of comparing the combined charge furthercomprises comparing a voltage at the common node to the reference.
 20. Amethod for analog-to-digital conversion in a multiple stage pipelinecomprising: at a first stage of the pipeline: storing an input chargethat corresponds to an analog input signal; a first comparing step ofcomparing a reference voltage against a voltage corresponding to thestored input charge; providing a first comparison signal in response tothe first comparing step; storing the first comparison signal to providea first digital output signal; transferring the stored input charge to asubsequent stage of the pipeline; redistributing a redistribution chargethat depends on the first comparison signal; and at the subsequent stageof the pipeline: a second step of storing the stored input charge asreceived from the first stage of the pipeline, receiving theredistribution charge from the first stage; a second comparing step ofcomparing a reference voltage with a combined charge corresponding tothe stored input charge and the redistribution charge, to provide asecond comparison signal; and storing the second comparison signal toprovide a second digital output signal.